The key factor to staying alert into old age is environment, not genetics - and that could lead to new techniques to help people stay sharp

Some people stay highly alert and intelligent into old age, whereas others are not so lucky.

But new research hints that luck - at least, genetic luck - has a smaller part to play than previously thought.

The most important factor in how much intelligence changes is environmental - in other words, it is something that we can control.

Age UK, which commissioned the intelligence research, taking in results from people who took tests at 11, and then again over 65, said that the news was positive - and could lead to new techniques to help people stay alert.

'It is incredibly positive as it suggests that we can have a real influence on how our brain ages through our lifestyle and other external factors,' said Professor James Goodwin of Age UK.

Researchers found genetic factors account for 24% of changes in intelligence over a person’s lifetime, meaning that the biggest effect was environmental.

The findings, published in the journal Nature, also suggest that many of the genes that affect intelligence in childhood also influence intelligence in old age.

Scientists in the UK and Australia combined DNA analysis with data from people who took intelligence tests at age 11, and again aged 65 to 79.

More than half a million genetic markers in around 2,000 people were studied.

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Professor Ian Deary, from the University of Edinburgh, said: 'Until now, we have not had an estimate of how much genetic differences affect how intelligence changes across a lifetime.

'These new findings were possible because our research teams were able to combine a range of valuable resources.'

'The results partly explain why some people’s brains age better than others. We are careful to suggest that our estimates do not have conventional statistical significance, but they are nevertheless useful because such estimates have been unavailable to date.'

Australian co-author Professor Peter Visscher, from the University of Queensland, said: 'Unique data and new genome technologies combined with novel analysis methods allowed us to tackle questions that were not answerable before.

'The results also strongly suggest how important the environment is helping us to stay sharp as we age.

'Neither the specific genetic nor environmental factors were identified in this research. Our results provide the warrant for others and ourselves to search for those.'

Professor James Goodwin, from the charity Age UK, which funded the study, said: 'This research is extremely exciting as it provides a greater understanding about why mental abilities change throughout our lifetime.

'It is also incredibly positive as it suggests that we can have a real influence on how our brain ages through our lifestyle and other external factors.

'The key now is to establish which lifestyle and environmental factors are most important so that we are able to do all we can to maximise our chances of ageing well.'

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Slowing down a bit? Don't blame your genes - your lifestyle is key to staying alert into old age